ubuysa
The BSOD Doctor
Many thanks for these suggestions. Would it be possible for the villain to install a programme that controlled the countdown and after a bit of searching around the hero manages to locate the programme and uninstall it just in time?
If you want to be semi-technical and have the hero stop the clock in the nick of time (and assuming it's a Windows laptop), you could have the hero work out that the bomb timer program was using the Windows Time service (that's built into Windows) for the countdown timing. The hero accesses the list of Windows services (the command services.msc will do that), finds the Windows Time service, sees that it's running and stops and disables the time service just in the nick of time, thus stooping the countdown. The hero can then uninstall the rogue program to make everything safe.
That's not actually what the Windows Time service is for, but it's close enough to reality for your target age group. You can have the hero groping around before realising that the Windows Time service might be the answer....